Health regeneration

Health Regeneration is a Champion statistic that defines the rate at which health is naturally restored over time. Health Regeneration is measured in-game by a stat called HP5 (Health Per 5 seconds). HP5 describes how many hit points are restored in a 5-second window. The actual healing effect from regeneration is applied every half-second, not every 5 seconds. This stat is similar to Mana Regeneration, although health regenerates more slowly than mana. Health regenerated over time is considered to be a form of healing for the purposes of healing increases or decreases, and for the sake of the "Healing Done" stat at the end of game screen. There are many items, abilities, and buffs which can increase HP5 either permanently or temporarily.

STPoacher's Knife: Passive – Jungler: Gain 10 health per second while under attack from neutral monster, 5 mana per second, and deal 45 magic damage on hit to monsters over 2 seconds. +30 bonus gold per large monster kill. Passive – Scavenging Smite: When you smite a large monster in the enemy jungle, you gain half a charge of smite. If you kill that monster, you gain +20 bonus gold, and you gain 175% increased movement speed decaying over 2 seconds. 850 .

STRanger's Trailblazer: Passive – Jungler: Gain 10 health per second while under attack from neutral monster, 5 mana per second, and deal 45 magic damage on hit to monsters over 2 seconds. +30 bonus gold per large monster kill. Passive – Blasting Smite: Smite deals damage in an area, dealing half damage to all monsters and enemy minions near the target and stuns them for 1.5 seconds. Casting smite on a monster restores 15% of missing health and mana. 850 .

STSkirmisher's Sabre: Passive – Jungler: Gain 10 health per second while under attack from neutral monster, 5 mana per second, and deal 45 magic damage on hit to monsters over 2 seconds. +30 bonus gold per large monster kill. Passive – Challenging Smite: Smite can be cast on enemy champions, marking them for 6 seconds. While marked, basic attacks deal true damage over 3 seconds, you have vision on them, and their damage to you is reduced by 20%. 850 .

Healing reduction effects reduce the effectiveness of regeneration. Such effects do not stack, though they will refresh the duration. All such effects (except Crystal Scar's Aura) halve the effectiveness of healing, health regeneration, life steal, and spell vamp.

Trivia

[Last updated 20/7/2015 on patch 5.13]

What may be the highest health regen attainable is 4790.052589 HP per 5 seconds, with Dr. Mundo thanks to his passive Adrenaline Rush and his ultimate Sadism which both scales with maximal health, so Dr. Mundo having a lot of health would make his health regen huge.